when send_guard is disabled, Entry should not be Send?

[skyzh]

For any circumstances, we always unsafe impl Send and Sync for Entry:

dashmap/src/mapref/entry.rs

Lines 139 to 144 in afe293f

	unsafe impl<'a, K: Eq + Hash + Send, V: Send, S: BuildHasher> Send for OccupiedEntry<'a, K, V, S> {}
	unsafe impl<'a, K: Eq + Hash + Send + Sync, V: Send + Sync, S: BuildHasher> Sync
	for OccupiedEntry<'a, K, V, S>
	{
	}

But within OccupiedEntry:

dashmap/src/mapref/entry.rs

Lines 133 to 137 in afe293f

	pub struct OccupiedEntry<'a, K, V, S> {
	shard: RwLockWriteGuard<'a, HashMap<K, V, S>>,
	elem: (*const K, *mut V),
	key: K,
	}

We have a RwLockWriteGuard. This should not be Send if send_guard is not enabled.

Maybe we should add a feature gate over the unsafe impl of Send and Sync for OccupiedEntry?

Thanks for investigating into this in advance!